The present invention relates generally to composite fabrication. More specifically, the present invention relates to composite fabrication employing tow-optimized designs. Advantageously, corresponding system and software programs for generating a tow-optimized composite structure are also disclosed.
Historically, research efforts in connection with composite manufacturing technology have focused on performance, rather than cost, considerations. This trend changed in the 1990""s when strict new cost guidelines were applied to emerging civil and military platforms, such as the Joint Strike Fighter.
The aerospace industry has responded to the low cost composites challenge by developing innovative manufacturing techniques, such as producing unitized parts with automated processes. The most significant technology promising reduced cost fabrication is the fiber placement process, which allows large, complex shaped composite structures to be produced faster, approximately 40% cheaper, and with greater quality than traditional approaches. Fiber placement has been used to manufacture military hardware such as the inlet duct of the Joint Strike Fighter (see the article by A. L. Velocci in Aviation Week and Space Technology (May 11, 1998. pp. 75-76) and the landing gear pod fairing of the C-17 transport (as discussed by V. P. McConnell in High Performance Composites (July/August, 1998. pp. 48-50)), as well as lighter aircraft for civil aviation (as mentioned in the report by E. H. Phillips in Aviation Week and Space Technology (Aug. 31, 1998. p. 39)). FIG. 1 depicts the inlet duct 5 of the Boeing Joint Strike Fighter (X-32) which duct is fabricated in accordance with the known fiber placement process.
Fiber placement is a modern, automated method of manufacturing a composite structure. This manufacturing method has received significant attention recently due to well-documented success in producing complex composite structures in a cost-effective manner. What is not well documented is the fact that the capabilities of existing fiber placement hardware far exceed the capabilities of current design engineering tools, particularly with respect to the ability to fabricate structures exhibiting steered or curvilinear fiber paths.
Fiber placement is a unique process combining the differential material payout capability of filament winding and the compaction and cut-restart capabilities of automatic tape laying. In the fiber placement process, narrow (xcx9c0.125 in.) strips or xe2x80x9ctowsxe2x80x9d of resin impregnated fiber are drawn under tension across a tool geometry by a computer controlled head. This head is capable of delivering up to approximately thirty adjacent tows simultaneously, allowing for high production rates. The narrow tows provide precise control over fiber orientation and, since each tow can be controlled independently, thickness tapers on complex geometry are readily produced. It will be appreciated that the control of fiber adds and cuts (the start and stop of individual tows) is controlled by a computer via a CAD interface.
FIG. 2 illustrates a plurality of feed paths employed in one layer of a composite structure in making a predetermined bend. From FIG. 2 it will be apparent that the feed rate of each tow is also individually controlled, allowing the longer path, i.e., the outside tows, of a steered radius to feed faster than the shorter, i.e., inside, path tows. The ability to support differential tow feed rates combined with the ability to drop individual tows provides the opportunity to place fibers along a relatively tight radius with no degradation in component quality. Fiber steering is made possible by local compaction during placement of the fibers, with each of the impregnated tows having enough tack to overcome any sliding forces.
It should be mentioned that when tows are steered through a radius, the fibers on the outside of the radius are placed in tension and the fibers on the inside of the radius are placed in compression. However, since the fibers are inextensible, the fibers along the inside radius can buckle if the steering is severe. Industry quality assurance programs have demonstrated that using fiber placement technology, carbon/epoxy fiber path geometry can be tailored to a maximum steering radius of twenty inches with no loss in specimen quality. See the discussion by B. Mcilroy in the xe2x80x9cFiber Placement Benchmark and Technology Roadmap Guidelines (Final Report),xe2x80x9d Air Force Research Laboratory contract F33615-95-2-5563 The Boeing Company, 1999). See also the article by R. Flory et al. entitled xe2x80x9cEffect of Steering and Conformance Requirements on Automated Material Deposition Equipment.xe2x80x9d (Charles Stark Draper Laboratory, Inc. technical capability document). Tighter steering radii are possible if the extent of the steering is not severe, e.g., if the arc radius extends less than forty-five degrees. In contrast, tape laying equipment, i.e., equipment performing another automated process utilizing single bands of material approximately six inches in width, is restricted to maximum steering radii in excess of twenty feet, or almost no steering.
It should also be mentioned that typical fiber placed parts might generate anywhere from 2% to 15% scrap, compared with 50% to 100% for conventional hand layup, as discussed by R. Aarns (The Boeing Company) during the Technical Contribution Award speech delivered at AIAA St. Louis Section Honors and Awards Banquet (20 May, 1999). This reduced material scrap rate directly equates to acquisition cost savings due to reduced material usage. Furthermore, the large unitized structures capable of being fabricated equate to life cycle cost savings due to reduced nonrecurring costs, hands-on labor, and part tracking. Finally, the automated process leads to increased accuracy (and, thus, improved quality) and reduced costs due to fewer processing errors and scrapped parts. Each of these advantages has propelled fiber placement into the spotlight. Thus, at the present moment, designs are being developed and produced throughout the aerospace industry, which designs are equivalent to conventional hand layup components, but at reduced cost.
In this development frenzy, a key advantage, i.e., the previously mentioned capability of fiber steering, is being largely overlooked. For example, fiber steering offers potential weight savings by overcoming the restriction of discrete linear fiber orientations commonly associated with traditional composites. More specifically, with conventional hand layup composites, one starts with tape or fabric plies of linear fiber orientation, and assembles these into desired stacks of laminate families, i.e., combinations of various orientations in a preferential stacking sequence. Within a given component, there are two predominant design conditions to consider: (1) overall laminate thickness required; and (2) the proper combination and stacking of various lamina orientations. To change either thickness or orientation requires a point discontinuity in the plies, which necessitates a ply termination at a boundary between adjacent regions of differing orientation. However, current analytical techniques focus on laminate optimization and not ply optimization, thus producing design concepts that are not optimized for either manufacturability or cost of production.
In order to create an efficient design for any component, the component design process must include a detailed consideration of the specific manufacturing processes involved. However, recently implemented, popular automated methods for producing composite structures have yet to develop and distribute the detailed process advantages and limitations in a format that is accessible to the design engineer. As such, there are many preliminary, and in some cases detailed, designs violate absolute requirements of the chosen manufacturing process. This absence of available information in the earliest stages of the design process necessitates redefinition of components, often several times. Furthermore, incorrect or incomplete information with respect to manufacturing requirements results in technically correct but irrelevant analyses to xe2x80x9coptimizexe2x80x9d intermediate designs that are simply not producible.
The fiber placement process, however, allows tailoring of the composite structure within a ply level by placing composite tows along curvilinear paths. This capability offers the potential for optimized structural configurations by tailoring fiber paths within a ply to load paths of the component. This capability was precluded with hand layup fabrication techniques due to the prohibitive cost of the required hands-on labor. In short, there is now the potential to produce reduced weight fiber steered components with no increase in manufacturing cost. It is time to recall that performance issues can still be addressed within the confines of low-cost composite processing.
There are no tools available for preliminary design to enable a composite designer to visualize actual tow geometry prior to fabrication. Manufacturers of fiber placement hardware (Ingersoll and Cincinnati-Milacron) do provide sets of simulation software (Acraplace and Offline Programmning System (OPS) software packages, respectively), but these require a complete definition of the ply boundaries to be manufactured as an input. In other words, these systems require a final detailed design before either quality or manufacturing concerns, or both, can be addressed. The designer""s only recourse, then, is to define the composite structures as though they would be fabricated by conventional hand-layup techniques. This approach is paramount to ignoring the details of the specific manufacturing process, resulting in ignorance of the limitations of the process and an inability to exploit the advantages of the process. This late consideration of manufacturability generally results in several test components being fabricated to work out the actual details of the tow paths, and necessary design changes. This sort of trial and error manufacturing is not cost-effective.
Furthermore, it should be noted that since the initial design assumed a conventional hand-layup, there is no valid technique for comparing an optimal hand-layup versus an automated layup configuration. The definition for automated tows and plies are necessarily tainted by incomplete and often inaccurate requirements associated with a manual manufacturing process. What is needed is a process and associated system for predicting, defining, analyzing and visualizing actual tow geometry, to include quantifiable computational assessments of common geometric flaws such as tow gaps and overlaps. This process and system would overcome the identified limitations, and has not previously been disclosed.
The fiber architecture and, more specifically, the detailed local material orientations and thickness, is required to accurately determine the mechanical properties of the composite. Complex geometry can result in fiber orientations and thickness build-ups that are not readily apparent. Complex curvature can, and most likely will, result in a fiber path that is also curvilinear. This will result in continuously varying mechanical properties, which must be accounted for to achieve an accurate analytical prediction of the mechanical response. There are no tools available to predict these local variations in fiber orientation, or the existence of significant overlaps or gaps in adjacent tows. These details are simply ignored in the analysis, or assumed by some sweeping generalizations.
It should be mentioned that Northrop-Grumman has developed a system with capabilities similar to a few of those functions discussed in detail below. More specifically, Northrop-Grumman has a system capable of visualizing tow geometry, assessing certain quality measure such as gaps and overlaps, and defining local fiber orientation. Note that many of the requirements for this were developed in discussions with the inventors of the present invention on a contract funded by the Office of Naval Research. See the report entitled xe2x80x9cFiber Steering for Reduced Weight Affordable Composite Structurexe2x80x9d (Contract N00140-95-2-J044 awarded to the Boeing Company by the Office of Naval Research Center of Excellence for Composites Manufacturing Technology (April 1997 to April 1998)). In particular, the concept of a closed-loop design process was a product of the inventors of the instant invention and other employees of The Boeing Company, and was partially disclosed to teammates on the referenced ONR contract. Northrop-Grumman apparently does not have a closed-loop system for design, analysis and manufacturing and does not have a system which will allow Northrop-Grumman to accurately assess the relative merits of various manufacturing methods. The definition of the Northrop-Grumman tow geometry may or may not be in true 3D space. In addition, the referenced system does not allow definition of the laminate solid, or documentation of the tows, plies, and cross sections.
Daimler-Chrysler has developed a system specifically tailored to the tape laying process. This system allows visualization of plies, but only after the detailed design definition. This system does provide some assistance for the development of the laminate solid, but in fact it treats plies in a manner opposite to that dictated according to the present invention. Daimler-Chrysler defines an ideal solid and then sections this solid to obtain apparent plies. The Daimler-Chrysler system apparently does not incorporate more than fundamental rules for manufacturability and does not provide the preliminary designer with any tools for evaluating design efficiency. Moreover, the system does not provide any visualization capability for a tow, does not provide any mechanism for identifying true local fiber orientation, and does not allow investigations into relative quality of alternate designs. The system also does not provide the capability for a comparative study between manual and automated manufacturing methods.
Vistagy, Inc. is currently developing a fiber placement interface for their FiberSim family of composite development tools. The fiber placement interface is intended to automatically generate fiber placement data files from within their CAD system directly from the 3D model of the composite part. As with their current Composite Engineering Environment, this differs from the process and system disclosed herein in that it requires a full 3D definition of the structure prior to the simulation for individual tows, and it does not fully account for tows in true 3D space. The process and system described herein encompass the process for preliminary design through detailed design and manufacturing, enabling design simulation of tows in 3D space concurrently with the development of the full 3D definition of the structure. It is also worth noting that individuals currently developing this software were briefed on technology relating to this disclosure under a nondisclosure agreement prior to May, 1998.
It should be apparent that if the individual pieces of the design, analysis, and testing methodology are not available, then clearly the process is not understood. There are no tools available to the composite designer that allows him/her to evaluate alternate steering patterns during either preliminary or detailed design. As such, current fiber placed components are designed in the same manner as conventional hand layup components. The details of the manufacturing process are not considered in the design stage, which necessarily limits the efficiency of produced designs. The limitations of existing composite design tools for complex fiber placed components has been experienced by the authors in applications to include defining the ply details for the inlet duct of the Boeing Joint Strike Fighter demonstrator (FIG. 1).
What is needed is a process that provides an integrated design for manufacturing/fiber steering capability for fiber placement that achieves optimum structural efficiency while producing affordable primary composite structures. What is also needed is analytical methodologies for steered fiber composites, linked to design tools which encompass the overall process flow and which allow parallel considerations for manufacturability and mechanical performance. It will be appreciated that this combination of tools will allow optimization for cost as well as weight. Thus, what is needed are tools and processes for design, analysis and automated manufacturing of composite materials and structures using techniques, e.g., fiber placement or tape laying, which integrate into an existing system and methodology which encompass a knowledge base for hand-layup manufacturing of composite materials and structures. The combination of these two techniques is clearly unique, and offers, for the first time, the capability to accurately assess competing methods of composite fabrication.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for tools and corresponding processes that overcome the above-described deficiencies. The present invention was motivated by a desire to overcome the drawbacks and shortcomings of the presently available technology, and thereby fulfill this need in the art.
The present invention offers significant increases in design and analysis capability for automated composite manufacturing processes such as fiber placement. This increased capability has the potential to offer significant weight savings in composite structural applications, at no additional cost. Other industries will realize the benefits of this research program as they explore the advantages of composite materials and begin to become involved with fiber placement.
According to one aspect, the present invention provides a composite design optimization process for designing a laminate part including steps for generating a globally optimized 3-D ply definition for a laminate part, optimizing the 3-D ply definition at the individual tow level, subsequently generating a feedback signal providing tow specific information, and modifying the 3-D ply and 3-D tow definition responsive to the feedback signal.
According to a further aspect, the present invention provides a laminate part constructed using a composite design optimization process for designing a laminate part comprising steps of generating a globally optimized 3-D ply definition for a laminate part, and optimizing the 3-D ply definition at the individual tow level, subsequently generating a feedback signal providing tow specific information, and modifying the 3-D ply and 3-D tow definition responsive to the feedback signal.
According to a still further aspect, the present invention provides a composite design optimization system used in designing a laminate part, comprising circuitry for generating a globally optimized 3-D ply definition for a laminate part, circuitry for optimizing the 3-D ply definition at the individual tow level, circuitry for subsequently generating a feedback signal providing tow specific information, and circuitry for modifying the 3-D ply and 3-D tow definition responsive to the feedback signal.